Covered storage tank with means to suspend floating roof when not in use

ABSTRACT

An enclosed storage tank having a bottom, side wall and fixedposition roof, a floating-roof in the tank, means to releasably secure the floating-roof, when not in use, in the upper part of the tank near and beneath the fixed-position roof, and access means from above and through the fixed-position roof to provide communication with the means to releasably secure the floatingroof so that the floating-roof can be secured in the upper part of the tank near the fixed-position roof or released to float freely on liquid in the tank. The access means can be a plurality of closeable ports in the roof.

United States. Patent n91 Strunc et a1.

[5 COVERED STORAGE TANK WITH MEANS TO SUSPEND FLOATING ROOF WHEN NOT NUSE [75] Inventors: Robert Winfield Strunc, Clarendon Hills; WrayBertram Young, Lombard, both Of 111.

[73] Assignee: Chicago Bridge & Iron Company, Oak Brook, Ill

[22] Filed: June 8, 1972 [21] Appl. NO.: 260,983

[52] US. Cl 220/26 R [51] Int. Cl B65d 87/18 [58] Field of Search220/26, 22.1, 88, 93

[56] References Cited UNlTED STATES PATENTS 1,700,887 2/1929 Glass220/26 R 2,586,856 2/1952 Orr et a1. 220/26 R 2,663,453 12/1953 Wigginset a1. 220/26 R 2,848,799 8/1958 Hanna 220/26 R X g [111 3,815,775 1June 11, 1.974

1/1959 Champagnat 220/26 R 4/1959 Wiggins 220/26 R PrimaryExaminer-William 1. Price Assistant ExaminerSteven M. Pollard Attorney,Agent, or Firm-Merriam, Marshall, Shapiro & Klose [5 7] ABSTRACT Anenclosed storage tank having a bottom, side wall and fixed-positionroof, a floating-roof in the tank, means to releasably secure thefloating-roof, when not in use, in the upper part of the tank near andbeneath the fixed-position roof, and access means from above and throughthe fixed-position roof to provide communication with the means toreleasably secure the floating-roof so that the floating-roof can besecured in the upper part Of the tank near the fixed-position roof Orreleased to float freely on liquid in the tank.

The access means can be a plurality of closeable ports in the roof.

11 Claims, 13 Drawing Figures PATENTEDJUH n m SHEEI 10F 5 fizzltf mmmlLT\ gm PATEWWM 1 m4 3; 15; W5 SHEEP 2 BF 5 PATENTEJUM 1 m4 sum 30$ 5Pmmsmunn m4 sgalsms SHEEI a 03- 5 This invention relates to storagetanks for liquids. More particularly, this invention is concerned withimprovements in enclosed storage tanks for liquids which employ aninternal floating roof.

Many kinds of storage tanks are widely used for the storage of liquidproducts. For reasons of cost and convenience of construction, largesize storage tanks are often made with a flat bottom and verticalcylindrical wall. The tank may be fully enclosed such as by the use of aconical or dome fixed-position roof. Alternatively, the tank can be leftopen at the top and a floating roof positioned inside of the tank tofloat on the liquid product stored therein'.

Although both described types of tanks are widely used, for the storageof liquids having a high volatile content it is beneficial to employ astorage tank which has an external fixed roof, such as of the conical ordome type, and an internal floating roof. The fixedposition roofprevents rain, snow and ice from entering the tank and thereby protectsthe contents from the weather and the internal floating roof greatlyretards the formation of vapor from volatile materials stored in thetank. By suppressing the formation of vapor, there is a greatly reducedneed to vent vapor from the tank during atmospheric changes intemperature and pressure. As a result there is a reduction in the amountof product lost through vapor venting.

Although a storage tank having a fixed external roof and an internalfloating roof can be used for storing volatile liquid products, itsometimes becomes advisable, and even necessary, for the tank to be usedfor the storage of a liquid product which is exceedingly low in volatilecomponents and often a product which is very viscous. It is undesirablein many instances to utilize the floating roof with such a productbecause it can be damaged through failure to operate in the same manneras it would when used in conjunction with the storage of a much morevolatile free-flowing liquid stored in the tank. Accordingly, forflexibility in use, there is a need for a storage tank for liquids whichcontains a floating roof which can be utilized when desired inconjunction with the storage of volatile free-flowing liquids and whichfloating-roof can be removed from service or use when the tank isemployed for the storage of a heavy or viscous liquid, or one low involatile components.

According to the present invention there is provided an enclosed storagetank comprising a tank having a bottom, side wall and fixed-positionexternal roof, a floating-roof in the tank, means to releasably securethe floating-roof, when not in use, in 'the upper part of the tankbeneath the fixed-position roof, and access means from above and throughthe fixed-position roof to provide communication with the means toreleasably secure the floating-roof so that the floating-roof can besecured beneath the fixed-position external roof or released to floatfreely on liquid in the tank.

In the enclosed storage tank of this invention, the floating-roofoperates in the conventional manner when employed in conjunction withthe storage of a volatile free-flowing liquid. However, when the storagetank is to be employed for storing a thick or viscous liquid, andparticularly one of low volatility, the floatingroof can be securedbelow the fixed-position roof above the usual maximum liquid storagelevel of product to be placed in the tank so that it thereby is removedfrom operating cooperation with the storage of the viscous liquid in thestorage tank. Securing of the floating roof as described can be achievedaccording to the invention, without a workman entering the tank, byfilling the tank with a liquid product to raise the floating-roof closeto the fixed-position roof. Then. by use of the access means, such asopenings or ports in the fixed-position roof, suspending means is put inplace to secure the floating-roof in nonfloating and nonoperatingposition. v

Various suspending meanscan be used to secure the floating-roof innonoperating position. Thus, the suspending means can comprise aplurality of rods, cables or chains connected at various points to thefloatingroof and also to the fixed-position roof or to other tankstructure above the floating roof. Furthermore, when internal columnsare present to support the fixedposition roof, locking means can bepositioned on the columns, or the floating-roof, to to lock thefloatingroof into position relative to the columns so that it can besupported or secured in position. Additional support can be provided bylocking means on the tank wall or the periphery of the floating-roofwhich functions to secure the floating-roof in releasable positionrelative to the tank wall. Such a wall-floating-roof securement can bethe sole means of supporting the floating-roof in nonoperating positionif the tank and floating-roof have the necessary strength. The lockingmeans can be positioned to be operable when the floating-roof is highinside of the tank and closely positioned by and beneath thefixed-position roof so as to haveit releasably placed in a location highenough so as not to interfere with use of the tank for storage of aviscous orlow-volatile liquid.

The invention will be described further in conjunction with the attacheddrawings, in which:

FIG. 1 is a plan view, partially broken away, of an enclosed storagetank according to the invention;

FIG. 2 is an elevational view, partially broken away and partially insection, of the enclosed storage tank of FIG. 1;

FIG. 3 is an enlarged vertical sectional view of the upper corner of thetank of FIG. 2 showing one part of a mechanism for suspending thefloating-roof;

' FIG. 4 is an elevational view, partially in section, taken along theline 4-4 of FIG. 3;

FIG. 5 is an elevational view, partially in section, of a vent mechanismlocated in the floating-roof deck;

FIG. 6 is a plan view, partially broken away, of the top of the columnsused to support the fixed-position roof of the tank of FIGS. land 2;

FIG. 7 is an elevational view, partially in section, taken along thelines 77 of FIG. 6;

FIG. 8 is an elevational view, partially in section, taken along theline 8-8 of FIG. 6;

FIG. 9 is an elevational view, partially in section, of the upper partof the central column in the tank of FIGS. 1 and 2;

FIG. 10 is a partial plan view taken along the line 10 10 of FIG. 9;

FIG. 11 is an elevational view, partially in section, taken along theline 11-11 of FIG. 9;

FIG. 12 is an elevational view, partially in section, of anotherembodiment of the invention in whicha floating roof is temporarily fixedin position inside of an enclosed storage tank having a fixed-positionroof supported without the use of internal columns; and

FIG. 13 is another embodiment of vent means which can be used in thefloating-roof to provide vapor communication from between the top andbottom of the floating-roof when it is placed in releasably fixedposition high inside of the tank.

So far as is practical, the same elements or parts which appear in thevarious views of the drawings, will be identified by the same number.

With reference to FIGS. 1 and 2, the enclosed storage tank has a flatmetal bottom 11, a vertical circular cylindrical side wall 12 and afixed-position conical roof 13 supported by wall 12 and also supportedby internal columns. Central column 14 supports the central area offixed-position roof 13. A plurality of columns 15, placed more or lessin a circular arrangement around central column 14, support anintermediate area of roof 13. Columns 16, also positioned in a generallycircular pattern inside of the tank support the roof area betweencolumns 15 and tank side wall 12. Girders 17 are positioned to extendbetween a pair of adjoining columns 15 and girders 18 are positioned toextend between a pair of adjacent columns 16. Rafters 19 rest on asupporting ring 93 (FIG. 9) on the top of central column 14 and extendto and rest on girders 17 and 18. Rafters 20 rest on girders 18 andextend to and rest on supporting flanges 21 positioned on the inside topedge of tank side wall 12. Roof l3 rests directly on the top of raftersl9 and 20. An enclosed storage tank of the type so far described isconventional in the art of tank construction.

v Located inside of tank 10 is a floating-roof 25. Floating-roof 25 hasa flatmetal deck 26 with a peripheral pontoon 27 located around itsouter edge. A flexible seal 28 is located on the outer edge of pontoon27 to seal the clearance between the pontoon edge and the inside surfaceof tank side wall 12.

The deck 26 of floating-roof 25 is provided with a plurality of openingsor holes through which the columns 14, 15 and 16 extend. Each hole issurrounded by a skirt 29 located on the upper side of deck 26. Each ofthe skirts 29 located around the holes in the deck extends verticallysufficiently high so that the liquid level therein does not extend abovethe top of the skirt when the floating-roof is floating on a liquid inthe tank.

Vent mechanism 30 (FIGS. 1, 2 and 5) is located in deck 26 of thefloating-roof. Vent mechanism 30 is closed when the floating-roof issupported by a liquid in the tank and is open when the floating-roof issuspended or held in fixed position high up inside of the tank when thefloating-roof is not to be used. Vent mechanism 30 is further describedherein in the discussion of FIG. 5.

Door is positioned over an access opening in roof 13 located above thetop of central column 14. Doors 41 are located over access openings inroof 13 above the top of columns 15 and doors 42 are located over accessopenings in roof 13 above the tops of columns 16. Doors 43 arepositioned over access openings in roof 13 positioned in spaced-apartrelationship around the.

peripheral portion of roof 13 to provide access to means to assist insecuring the floating roof in position when it is not to be used.

With reference to FIGS. 3 and 4, rod 50 is joined to flange 51 on thefloating roof by means of pin 52. The upper end of rod 50 is joined toflange 53 on rafter 20 by means of a bolt or pin 54. A plurality of rodsare spaced around the periphery of the floating-roof and fixed-positionroof. A plurality of spaced-apart openings in roof 13 are provided sothat access to rods 50 can be achieved by a man on the fixed-positionroof 13. Generally an opening is provided directly above each rod tomake access easy. A vertical wall 56 is positioned around each accessopening 55 to keep water from running into the access hole. Door 43seals off the access area and rests on vertical wall 56 when in closedposition. Door 43 is mounted on wall 56 by means of a hinge arrangement57.

FIGS. 6 to 8 illustrate the construction of and on the upper part ofcolumns 15. As shown in these figures collar 61 is secured to the upperpart of column 15. The ends of girders 17 are joined to flanges 62 oncollar 61 by plates 63. Rafters 19 rest on the top of girders 17 andfixed-position roof 13 in part rests on and is supported by rafters 19.An access opening 65 is located in roof 13 above each of the columns 15.Each opening 65 is surrounded by a vertical cylindrical flange or wall66 which is covered by door 41 movably secured to flange 66 by hinge 67.

Also in reference to FIGS. 6 to 8, a skirt 29 surrounds each opening infloating-roof deck 26 through which each column 15 projects. Each skirt29 has a stilling well wall 71 and a column well wall 72 which arejoined together by horizontal plate ring 73. In practice, when thefloating-roof is supported by liquid in the tank, the liquid levelshould not rise above the horizontal ring 73 at the top of the stillingwell space.

As shown in FIGS. 6 to 8, a pair of tension rods 75 in part support thefloating-roof on each of columns 15 when the floating-roof is not inuse. Each rod 75 has a lower portion 76 and an upper portion 77 witheach rod portion being joined together by a pin 78. The lower ends ofeach rod portion 76 are joined by a pin 79 to flanges 80 joined tofloating-roof deck 26 and stilling well wall 71. The upper ends of eachupper rod portion 77 are joined by pins or bolts 81 to flanges 82mounted on the top portion of column 15. The hinge joint or knucklejoint provided by pins 78 in rods 75 facilitates the installation andremoval of pins.8l when the floating-roof is to be suspended in the topof the tank or when it is to be removed from such suspension. It is ofcourse obvious that a similar rod construction can be used for rod 50 asillustrated in FIGS. 3 and 4.

Although the construction as illustrated by FIGS. 6 to 8 pertainsspecifically to the apparatus at the upper part of columns 15, it shouldbe understood that the apparatus at the top of columns 16 is in allrespects the same as is shown in FIGS. 6 to 8 and therefore has not beenseparately illustrated or described.

FIGS. 9 to 11 illustrate the construction at the upper portion ofcentral column 14. Circular plates 91 are secured to the top of column14. Beams 92 are positioned to rest on the top of plates 91 and toextend radially outwardly therefrom to join with and support rafter ringsupport 93. The inner ends of rafters 19 rest on rafter ring support 93.Circular plate 94 is joined to the tops of radially positioned beams 92and is supported thereby. Vertically positioned flanges 95 are securedto plate 94. The upper end of suspension rods are joined by bolts 101 toflanges 95 and the lower end of suspension rods 100 are joined by pins102 to flanges 103 connected to the skirt 29. Each of the two suspensionrods 100 is made in two parts or portions 105 and 106 which arepivotably joined together by pins 104. The phantom view A shown in FIG.9 illustrates how the suspension rods 1111) can be folded out of the waywhen the floating-roof is not suspended from the upper part of the tankin the manner described.

As shown in FIG. 9, opening 110 is provided in fixedposition roof 13above the top of column 14. Vertical circular cylindrical wall 111 islocated around the opening 110 to prevent water from flowing into thehole. Door 4% is mounted by hinge arrangement 112, as shown in FIG. 10,to cylindrical wall 111 to movably cover the access opening. To obtainaccess to the upper part of column 14 to position suspension rods we inplace or to loosen them, door 40 need only be lifted upwardly.

The operation of the enclosed storage tank as described with referenceto FIGS. 1 to 11 is conventional except for the means to support thefloating-roof high up inside of the tank when the floating-roof is notto be used. As shown in FIG. 2, when the liquid level in the storagetank is at level L1 the floating-roof will be in the position shown inphantom floating on liquid in the tank. The normal maximum height ofliquid stored in the tank is at the line L2. The liquid level in thetank is accordingly not raised above the level line L2 when thefloating-roof is in use. Of course, when the floatingroof in in use thesuspension rods 50, 75 and 100 are released at their upper or lower endsor both so that there is unrestricted freedom of movement of thefloating-roof with rise and fall of liquid in the storage tank.

When it is desired to take the floating-roof out of use, the liquid inthe storage tank is raised to level L3. The liquid is maintained at thelevel L3 with the floatingroof on top of the liquid. Workmen then mountthe fixed-position roof 13, open the doors 40, 41, 42 and 43 and securethe suspension rods 50, 75 and 100 into place. After all of thesuspension rods have been secured in place, liquid in the tank iswithdrawn and the floating-roof is thereby left in secured position inthe upper part of the storage tank. After all of the contents of thetank has been removed, a different liquid can be fed into the tank andstored therein, it being assumed that the liquid fed therein isdesirably stored in the tank without use of the floating-roof on theliquid surface. When it is again desirable to place the floating-roof inuse, the quantity of liquid in the storage tank is raised to level L3 toplace the floating-roof in a slightly floating position on the liquid.Workmen can then release the suspension rods 50, 75 and 100 by openingdoors 40, 41, 42 and 43 and releasing the rods from their suspendingarrangement without entering the tank.

FIG. 5 illustrates a vent mechanism which can be positioned in deck 26of floating-roof 25. Opening 120 is provided in floating-roof deck 26.Vertical circular cylindrical wall 121 surrounds opening 120 andsupports a horizontally positioned plate ring 122 from which acylindrical circular collar 123 projects vertically upwardly. A pair ofrods 124 and 125 depend downwardly from the bottom of horizontal ring122. The lower ends of rods 124 and 125 are maintained spaced apart byconnecting rod 126. Float 127 is secured on diametrically opposite sidesby guides 128 and 129 through which vertical rods 124 and 125 slide in amanner which permits vertical displacement of float 127 up and down.When the floating-roof is supported by a liquid stored in the tank,float 127 is pressed against the lower internal edge of vertical collar123 to thereby seal off the passage in collar 123 to prevent flow ofvapor from the liquid beneath the floating-roof to the space above thefloating-roof. When the floating-roof is not in use and is secured inthe upper part of the tank, float 127 drops downwardly to position Bshown in phantom in FIG. 5 and thereby opens'the passage in collar 123so that any vapor beneath the suspended floating-roof and above theliquid level in the tank can be vented to the space above the securedfloating-roof. The vapor can then be removed from the space above thesecured floating-roof by means of a suitable conduit, not illustrated,in fixed-position roof 13.

FIG. 13 illustrates another vent mechanism which can be used to achievethe same results described with respect to the vent mechanism embodimentof FIG. 5. As shown in FIG. 13, vertically-positioned cylindricalstilling well wall 130 is located around opening 131 in floating-roofdeck 26 Horizontally positioned ring plate 132, having a centralcircular hole therein, is mounted on the top of stilling well wall 130.Cylindrical circular wall 133 is mounted on top of horizontal plate 132with its lower end positioned around the opening in plate 132. Arm 134extends outwardly and upwardly from the upper part of cylindrical wall133. Ann 135 is pivotally mounted by pin 136 to arm 134. A circular flatplate 137 is joined to arm 135 in a manner which permits it to cover thetop of cylindrical wall 133 to seal off flow of vapors from beneath thefloating-roof to the space above the floating-roof when thefloating-roof is in-use floating on a stored liquid. Vertical leg 138 isjoined to arm 135 by suitable pins or bolts placed in holes spaced apartto provide for adjustable positioning. Roller 139 is mounted at theupper end of leg 138. When the floating-roof is raised to level L3, asshown in FIG. 2, roller 139 contacts the bottom surface offixed-position' roof l3 and is caused to roll along such surface to theposition shown in phantom in FIG. 13. This results in pivotal movementof leg 138 which causes arm 135 to rotate and to be raised and, in doingso, cover 137 is lifted off of the top of cylindrical wall 133 tothereby provide an open passage for flow of vapor from beneath thesecured floating-roof to the space above-the said roof. After thefloating-roof is released from its secured position and the liquid levelin the tank is lowered, the weight of cover 137 causes it toautomatically reposition itself on top of cylindrical wall 133 to sealthe vapor passage.

FIG. 12 illustrates application of the invention to an enclosed storagetank 140 which has no internal col umns supporting the fixed-positionroof. The tank 140 has a flat metal circular bottom 141, a verticalcircular cylindrical metal side wall 142 and a conical fixedpositionmetal roof 143 supported by side wall 142 and rafters 146. Inside of thetank is located floating-roof 143 which has a metal deck 144, aperipheral pontoon 145 and a peripheral seal 149. When the floating-roofis not in use, it is suspended from rafters 146 by means of suspensionrods 147. Each of the suspension rods 147 is provided with a pivotableconnection intermediate its ends in the manner of suspension rods 100previously described. The lower ends of suspension rods 147 arepivotably joined to connecting flanges mounted on floating-roof 143. Theupper ends of suspension rods 147 are removably connected to flangesmounted on rafters 146. An access door 148 is located over an opening,in fixedposition roof 143, above each of the suspension rods 147. Uponopening of each and every access door, a workman can reach in to connector disconnect the suspension rod upper end from the connecting means onrafters 146. When the floating-roof is in use floating on a liquidstored in the tank, the suspension rods 147 are left connected theretoby the bottom pivotable connection, as shown in phantom in FIG. 12, sothat they can be readily reconnected to secure the floating-roof inposition high up in the tank when it is not to be used.

Although the invention has been described with reference to tanks havingvertical circular cylindrical walls, it will be obvious that theinvention can be utilized in tanks of different shapes including thosehaving polygonal sides. In addition, the fixed-position roof need not beconical but can be any other suitable shape including a domed shapedfixed-position roof. Furthermore, while the invention has been describedwith reference to tanks made of metal, it should also be understood thatthe invention can be used in tanks of concrete and plastic construction.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. An enclosed storage tank comprising:

a tank having a bottom, side wall and fixed-position roof,

a floating-roof in the tank,

means to releasably secure the floating roof, when not in use, onto tankstructure in the upper part of the tank near and beneath thefixed-position roof, and

access means from above and through the fixedposition roof to providecommunication with the means to releasably secure the floating-roof sothat the floating-roof can be secured in the upper part of the tank nearthe fixed-position roof, or released to float freely on liquid in thetank, by a man without him entering the tank.

2. An enclosed storage tank according to claim 1 in which the accessmeans comprises a plurality of closeable ports in the roof.

3. An enclosed storage tank according to claim 1 in which thefixed-position roof is supported by a plurality of columns extendingfrom the tank bottom to the fixed-position roof, and the floating-roofhas an opening therein for each column, each opening having a verticalskirt extending above the level of any liquid upon which thefloating-roof floats.

4. An enclosed storage tank comprising:

a tank having a bottom, side wall and fixed-position roof,

a plurality of vertical columns extending from the tank bottom to theroof to support the fixedposition roof,

a floating-roof in the tank,

an opening in the floating-roof for each column to extend therethrough,

a vertical skirt above each opening extending above the level of anyliquid upon which the floating-roof means to suspend the floatingkoof,when not in use,

from fixed-position tank structure above the floating-roof,

access means from above and through the fixed position roof providingcommunication with the means to suspend the floating-roof so that thefloatingroof can be so suspended or released from such suspension by aman without him entering the tank.

5. An enclosed storage tank according to claim 4 in which the means tosuspend the floating-roof is located at least in part in close proximityto the top of some of the columns.

6. An enclosed storage tank according to claim 5 in which the accessmeans includes at least in part closeable openings adjacent the tops ofcolumns having suspending means for the floating-roof.

7. An enclosed storage tank according to claim 4 in which the means tosuspend the floating-roof comprises elongated tension members.

8. An enclosed storage tank comprising:

a tank having a bottom, side wall and fixed-position roof,

a floating-roof in the tank,

a plurality of spaced-apart rods extending from the floating-roof totank structure above the floatingroof to releasably secure the floatingroof, when not in use, in the upper part of the tank near and beneaththe fixed-position roof, and

access means from above and through the fixedposition roof to providecommunication with the rods to releasably secure the floating-roof inthe upper part of the tank near the fixed-position roof or to release itto float freely on liquid in the tank.

9. An enclosed storage tank according to claim 8 in which the rods aredetachably connected at at least one end to the floating-roof or to tankstructure above the floating-roof.

10. An enclosed storage tank according to claim 8 in which at least someof the rods are of variable length.

11. An enclosed storage tank comprising:

a tank having a bottom, side wall and fixed-position roof,

a floating-roof in the tank,

means to releasably secure the floating-roof, when not in use, in theupper part of the tank near and beneath the fixed-position roof,

access means from above and through the fixedposition roof to providecommunication with the means to releasably secure the floating-roof sothat the floating-roof can be secured in the upper part of the tank nearthe fixed-position roof or released to float freely on liquid in thetank, and

a vent in the floating-roof which is closed when the floating-rooffloats on liquid in the tank and is open when the floating-roof isreleasably secured onto tank structure so that vapor can flow from oneside of the floating-roof to the other side.

1. An enclosed storage tank comprising: a tank having a bottom, sidewall and fixed-position roof, a floating-roof in the tank, means toreleasably secure the floating roof, when not in use, onto tankstructure in the upper part of the tank near and beneatH thefixed-position roof, and access means from above and through thefixed-position roof to provide communication with the means toreleasably secure the floating-roof so that the floating-roof can besecured in the upper part of the tank near the fixed-position roof, orreleased to float freely on liquid in the tank, by a man without himentering the tank.
 2. An enclosed storage tank according to claim 1 inwhich the access means comprises a plurality of closeable ports in theroof.
 3. An enclosed storage tank according to claim 1 in which thefixed-position roof is supported by a plurality of columns extendingfrom the tank bottom to the fixed-position roof, and the floating-roofhas an opening therein for each column, each opening having a verticalskirt extending above the level of any liquid upon which thefloating-roof floats.
 4. An enclosed storage tank comprising: a tankhaving a bottom, side wall and fixed-position roof, a plurality ofvertical columns extending from the tank bottom to the roof to supportthe fixed-position roof, a floating-roof in the tank, an opening in thefloating-roof for each column to extend therethrough, a vertical skirtabove each opening extending above the level of any liquid upon whichthe floating-roof floats, means to suspend the floating-roof, when notin use, from fixed-position tank structure above the floating-roof,access means from above and through the fixed position roof providingcommunication with the means to suspend the floating-roof so that thefloating-roof can be so suspended or released from such suspension by aman without him entering the tank.
 5. An enclosed storage tank accordingto claim 4 in which the means to suspend the floating-roof is located atleast in part in close proximity to the top of some of the columns. 6.An enclosed storage tank according to claim 5 in which the access meansincludes at least in part closeable openings adjacent the tops ofcolumns having suspending means for the floating-roof.
 7. An enclosedstorage tank according to claim 4 in which the means to suspend thefloating-roof comprises elongated tension members.
 8. An enclosedstorage tank comprising: a tank having a bottom, side wall andfixed-position roof, a floating-roof in the tank, a plurality ofspaced-apart rods extending from the floating-roof to tank structureabove the floating-roof to releasably secure the floating roof, when notin use, in the upper part of the tank near and beneath thefixed-position roof, and access means from above and through thefixed-position roof to provide communication with the rods to releasablysecure the floating-roof in the upper part of the tank near thefixed-position roof or to release it to float freely on liquid in thetank.
 9. An enclosed storage tank according to claim 8 in which the rodsare detachably connected at at least one end to the floating-roof or totank structure above the floating-roof.
 10. An enclosed storage tankaccording to claim 8 in which at least some of the rods are of variablelength.
 11. An enclosed storage tank comprising: a tank having a bottom,side wall and fixed-position roof, a floating-roof in the tank, means toreleasably secure the floating-roof, when not in use, in the upper partof the tank near and beneath the fixed-position roof, access means fromabove and through the fixed-position roof to provide communication withthe means to releasably secure the floating-roof so that thefloating-roof can be secured in the upper part of the tank near thefixed-position roof or released to float freely on liquid in the tank,and a vent in the floating-roof which is closed when the floating-rooffloats on liquid in the tank and is open when the floating-roof isreleasably secured onto tank structure so that vapor can flow from oneside of the floating-roof to the other side.